This invention is related to fabrication of a detection or sensing probe unit and particularly to such a sensing probe unit which is particularly useful for detecting environmental borne constituents generated as a result of combustion, pollution or the like.
Combustion detection and alarm systems employing various sensing and detecting means have been suggested, such as thermal, flame, photo-electric, ionization chamber, semiconductors of a metal oxide or polymeric organic material, and electrolyte cell sensors. A particularly satisfactory apparatus is disclosed in the copending application of the present inventors entitled Charged Non-Conductive Polar Sensing Element and Detection System filed on Oct. 11, 1978 with Ser. No. 950,413. The system disclosed therein includes an improved sensing element which includes a sensitive charged non-conductive sensing layer affixed to a metal plate which also serves as the terminal means for connection with a detection circuit. The sensing layer is preferably a polymeric material which has a surface resistivity in excess of 1.times.10.sup.10 ohms/square at 50% RH (relative humidity) and a bulk resistivity in excess of 1.times.10.sup.12 ohm cm at 50% RH and which is essentially free of polar substances. The layer includes either an internal and/or surface charge to establish a high intensity electric field. The electret sensing element, either connected as an electrostatic probe or as one plate of a capacitor sensing unit, provides practically instantaneous response to the presence of products of combustion. The electret sensing element will sense early generated aerosol components such as toxic polar gases as well as the particulate and charged particles, all of which may occur in different degrees at the incipient stage of combustion depending upon the particular combustion materials. Electrets are known elements and have been formed in various procedures.
As disclosed in the above application, the preferred polymeric electret materials generally include polytetrafluoroethylene (Teflon TFE), polystyrene, and polyethylene, all of which have a dipole-hydrogen bonding force value of surface energy of less than 1 ergs/cm.sup.2. The electret includes a detection surface which forms part of a free space exposed to the products of combustion at the incipient stage of combustion and in particular, including the toxic and noxious gases produced by the process of combustion. The detection appears dependent upon the ability of the electret surface to adsorb polar gas molecules generated by combustion, with the charged probe producing significant amplification in response to the resulting induced charge which can be detected. The charged probe or electrode also has the ability to attract charged ion radicals, aerosols, polar gas molecules and the like associated with products of combustion and the like which move into close proximity to the surface.
Although a sensitive probe is created, the magnitude of the charge is generally such that a high input impedance device which also has good electrometer characteristics is necessary to detect the signal. Generally, a FET (field effect transistor) detecting and amplifying unit provides a highly satisfactory system as shown in other prior art such as U.S. Pat. Nos. 3,754,219 and 3,989,463.
An electret is therefore defined herein as any dielectric or insulating material having a bulk resistivity of at least 1.times.10.sup.12 ohm cm and a surface resistivity of 1.times.10.sup.10 ohms/square, and which produces a permanent high intensity electric field as a result of an electric charge essentially permanently affixed to the electret, and which charge may be on or in the dielectric material. Thus, the electric field may result either from the appropriate alignment of the internal dipoles in the dielectric material or as the result of an electric charge deposited within or on the surface of the electret. Generally an electret material may be considered the electrostatic analogy of a permanent magnet, with the exception that the electret material may contain both electric dipoles and electric monopoles. This fundamental difference provides the basis for polarization of dielectric material to form an electret in various different methods and generally a greater number than that permitted in the polarization of a magnetic material. Thus, in the case of a dielectric material, the necessary charge and polarization may be created either by an applied electric field, by either electronic, ionic, or orientational polarization, or charge implanting. An electric field applied to the dielectric may generate relative displacement of positive and negative charges within an atom of the dielectric material, resulting in the induction of a dipole moment within the atom which originally had no charge or dipole moment. In addition, positively or negatively charged ions may be displaced relative to each other by the action of a field, thereby generating ionic polarization within the material. Such polarization however, is lost upon removal of the electric field. Various dielectric materials include molecules which have a permanent dipole moment and when an external electric field is applied to such material, the elementary molecules or portions of them tend to align with the dipoles in the direction of the field. When the field is removed, the molecules may remain in the aligned position within the dielectric material. Such dielectric materials having an internal field as a result of such dipole orientation are defined as including a heterocharge because the polarity of the surface charge is opposite that of the polarizing electrode. In addition to dipole moment alignment, charged particles or electrons can be introduced into the dielectric material or on the surface from an external source. Such internally generated charges have been deposited using a corona discharge or an electron beam. Such an externally charged electret, is defined as having a homocharge because the charges are implanted, have the same polarity as the polarizing electrode, and are all of the same charge.
The fabrication of the sensor element requires the intimate attachment of the electret to the compensating electrode as well as charging of the dielectric body to create the electret with the necessary field intensity over a substantial period of time.